Porous wall construction



March 3, 1964 H. L. WHEELER, JR

POROUS WALL CONSTRUCTION Filed Aug. 25. 1958 mm k AY" "188mm s" "Hmmmvmut`s""` 2710.3 2

United States Patent C) 3,123,446 PGRUS WALL CONSTRUCTIN Harry L.Wheeler, lr., La Canada, Calif., assignor to California instituteResearch Foundation, Pasadena, Calif., a corporation of California FiledAug. 2S, 195e, Ser. No. 756,912 9 Clain (fil, 2-9-183) This inventionrelates to porous wall construction and is a 'continuation-'impart of mycopending application Serial No. 335,670, filed February 9, 1953, nowabandoned, for Porous Wall Construction ,and Method of Manufacture.included in the objects of my invention First, to provide a porous wallwhich may be made tubular in form or tapered or which may be reformedfrom such initial shapes into asymmetrical form or cut and reformed intoa sheet.

Second, to provide a porous wall construction wherein the pore spacesmay be accurately controlled so that not only is the construction ofuniform porosity, but the size, direction and tortuousness of the porespaces may be accurately predetermined so as to meet specific needs;that is, the pores may be normal to the wall surface, inclined, varyingin area, or of a labyrinth nature.

Third, to provide a porous wall construction which may have a controlledvariation in porosity to predetermine relative flow of fluids withindifferent areas.

Fourth, to provide a porous wall construction through which a fluid maybe introduced to serve as a coolant or a fuel and in either casemaintain a protective boundary layer.

Fifth, to provide a porous wall construction which may be madesufficiently thin and large enough in area to permit its use as anaerodynamic surface, to serve, for example, for boundary layer controlor removal.

Sixth, 'to provide a porous wall construction which may be arranged tofunction as an effective filter.

Seventh, to provide a porous wall wherein a plurality of layers orlamin-ations of wires are formed on a mandrel by wrapping wire undertension helically back and forth along a mandrel, whereupon the wire isheated to fuse and bond the wire at cross points, the mandrel beingtreated to avoid bonding with the wire, and wherein the body formed rbythe wire is then removed from the mandrel and pressed between rollers orthe like to compress the wires comprising the wall, particularly at andadjacent the cross points, and whereupon the wire body is again heatedto effect further fusion and bonding of the Vflli.

With the above and other objects in View' as may appear hereinafter,reference is directed to the accompanying drawings in which:

FIGURE 1 is an enlarged fragmentary plan view showing a pair oflaminations of a typical porous wall constructed according to thisinvention.

FlGURES 2, 3 and 4 are fragmentary sectional views taken tln'ough 2 2,3--3 and tr- 4, respectively of FlGURE l.

FIGURE 5 is a greatly exaggerated, fragmentary' plan' view showing oneform of porous wall constructed according to this invention. j

FlGUE 6 is an idealized, sectional View thereof taken through 6--6 ofFIGURE 5 `FEE- EURE 7 is a similar sectional view thereof taken through7 7 of FIGURE 5.

FGURE 8 is an exaggerated, fragmentary, plan" View taken substantiallyalong the line S-S of FlGURE 9 showing successive layers of another formof the porous wall construction.

FIGURE 9 is a transverse', sectional view thereof through 9--9 of FIGURE8.

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FIGURE 10 is an exaggerated, sectional view similar to FIGURE 6 byshowing a modified form of the porous wall vconstruction in which theporosity varies between the two surfaces of the wall construction.

FIGURE 11 is a fragmentary, sectional View indicating diagram-medicallyan arrangement of pore spaces tending to induce tangential flow offluid.

FIGURE 12 is a diagrammatical, sectional `view of the wall constructionlas it :appears when flat and arranged with the pore spaces slopingrelative to the surfaces of the wall construction.

FIGURE 13 is a diagrammatical View showing two layers of wire comprisingthe wall construction in juxtaposition before heating and bonding.

FIGURE 14 shows these wires as they bond together in the course ofheating.

FIGURE l5 is a fragmentary, sectional View similar to FIGURE 14 showingthe wires coated or plated.

FIGURE 16y is a similar view showing the coated wires after bondingwherein the coating of adjacent wires is fused together.

ln the exercise of this invention, one or more fine wires 1 are woundunder tension on to a mandrel having the shape of the finished object ora shape from which the finished object can be developed. Essentiallysuch a mandrel is a figure of revolution such as a cylinder, cone, ormore complex shape such as a venturi throat. By controlling the mannerin which the wire is wound on the mandrel and by controlling the crosssection of the wire itself, a porous wall may be constructed, theporosity of which may be uniform, or, if desired, non-uniform, orgraduated, but in any case the porosity is predictable to a closedegree.

The wire Iis wound on a mandrel capable of withstanding the bondingtemperature of the material comprising the wire' l. Thus the mandrel maybe formed of ceramic material lor formed off metal and Ycoated with aceramic material or otherwise treated so that its surface will not bondwith the wire 1. After the wire has been wound to `form a plurality of'laminations the mandrel with the wire thereon, is subjected to bondingtemperatures lcausing the contacting surfaces of the laminations of Wireto' zfuse together under the influence of pressure resulting fromtension.

While the wire may have a wide Variety of cross sectional conligurationssuch as circular, rectangular, or ellipsoifdal' cross section, it hasbeen found that a relatively flat wire is advantageous; for example, awire flattened to ya thickness-to-width ratio of two to one or more.This may be accomplished by running the wire between rollers eitherprior to winding or in the course of the winding the wire on themandrel. The flat configuration provides a maximum area over which thesuccessive laminations of wire are in pressure Contact and thus insuresan adequate area to be bonded when heated.

The cross sectional area of the wire l varies with the intended use andsize of the mandrel on which it is wound. The larger the' mandrel themore coarse the wire which may be used. However, in any case extremelyfine wire may be used. For example, wire flattened to one thousandth inthickness has been used with success.

It has been found feasible first to provide an initial bond, between thewires, by heating the wires while on the mandrel, then remove themandrel, thenv subject the porous wall formed by the wire to mechanicalpressure, and then reheat and complete the bond. Also, the wire may be'wound' in cylindricalform then pre-bonded, then removed ,from themandrel and slit lengthwise, then flattened into a sheet, and thenpassed between rollers, and then againv heated to increase the bond.

Inasmu'ch as the wire is wound under highy tension, high radial pressureis exerted between the wires at the points of crossing and is maintainedin the completely wound structure while it is still on the mandrel. Theinitial bonding is accomplished by heating the wound wire while still onthe mandrel and the pressure exerted by wire tension provides therequired pressure for eecting a secure bond at the temperaturesemployed. After the structure is removed from the mandrel and compressedto a finer dimension, the portions of the wires adjacent the initialbond are deformed and the area of contact between the wires is extendedor increased. Since the wire is plastically deformed and by reason ofsuch deformation pressure exists between the wire surfaces at theextended areas of contact and the final heating results in fusing thewires, by heat and the then existing pressure, to provide the requiredbond. During the compression of the porous wall after removal from themandrel, the wires of the laminations are forced closer together, thusproducing passages of even smaller dimension than existed at the startof the rolling and permits construction of porous walls with extremelyfine passageways therethrough and permits accurate control of theuniformity and porosity of the product.

Thus, the porosity is not only controlled by the initial spacing anddimension of the wires as wound but also by the degree of compression.if relatively light rolling pressure is exerted by the rollers, maximumporosity for a given initial wire spacing is obtained; whereas if aheavy rolling pressure is exerted the porosity is correspondinglyreduced.

While the major rolling pressure is preferably applied after removalfrom the mandrel, an initial rolling operation may be performed afterthe porous wall structure is sintered but while the porous wallstructure is still on the mandrel. This operation tends to loosen theporous wall structure from the mandrel. lf a thin ceramic coating isutilized on the mandrel as a parting agent, it may be crushed by suchinitial rolling and thus further facilitate removal of' the porous wallstructure.

As has been indicated hereinbefore, the spacing between individualconvolutions of wire and the relative position of the wires comprisingsucceeding convolutions makes possible the construction of a porous wallhavhig precisely the selected porosity and resistance to fiow; that is,with a given porosity the resistance to flow may be increased ordecreased depending on the relation between succeeding laminations ofthe wire.

Whether a single wire or a multiplicity of wires are used, the windingpitch is such that the wires of succeeding layers cross at a substantialangle With a result that the resulting wall structure, will havefavorable strength longitudinally as well as circumferentially.

lf a single wire is used, it follows that many traverses of the wire arerequired before the spaces between the initial convolutions of the wiremay be filled. This, of course, results in crossing of the wires atperiodic points along the figure of revolution as shown in FIGURES 1through 6. Any increase in thickness at these points is eliminated afterthe initial heating and bonding of the wires by pressing or running thewall structure between rollers.

More specifically, in the course of filling in or completing onelamination designated 2 in FIGURES 1 through 4 by repeated traverses ofthe wire, a second lamination designated 3 is also completed, due to thefact that for each forward traverse of the wire there is a returntraverse. Each lamination corresponds to a row of wires as shown inthese figures, that is, the first lamination 2, corresponds to thebottom row in these figures and the second lamination 3 corresponds tothe second row.

The cohelical sets of wires comprising, respectively, the sum of theforward traverses and the sum of the return traverses are not identicalwith the first and second laminations 2 and 3, particularly after thewall structure thus formed is compressed; that is, the portions of thewires comprising the forward cohelical set of wires overlie the returncohelical set of wires and are thus pressed into the second lamination,and similarly portions of the return cohelical set of wires are pressedinto the first lamination. The depressing operation deforms each wirewhere it shifts from one to the other lamination as indicated by d,resulting in increased bond adjacent at these spaced points after thefinal heating and bonding step. As the winding of the wire is continued,additional pairs of laminations are formed.

As mentioned hereinbefore, the wire may be wound on itself to produce awide Variety of porous conditions. Perhaps the simplest form involveswinding the wire at a uniform rate back and forth over a rotatingcylinder. By controlling the relative rate of rotation and feed, thesucceeding laminations of the wire may be stacked in such a way thatvertical pores may be formed. Thus, as shown in FIGURES 5, 6 and 7, thewire 1 may be wound to form channels or passages 5 parallel to eachother and to the surfaces of the wall thus formed. If the wires are laiduniformly they may define ports 6 which extend normal to anduninterrupted between the two surfaces of the resulting cylinder orother figure of revolution. The porosity is controlled by the closenesswith which the wires are wound, as well as lateral fiattening as mayoccur when the wall is compressed.

The manner in which the wire may be fed on to the rotating mandrel maybe so regulated that succeeding layers do not place the ports inalignment. Thus, as shown in FIGURE 8 if the succeeding layers arestaggered, the ports 6 are not connected except through the channels 5.The result is that a multiplicity of labyrinth passages are formed,which, however, may be uniformly spaced and quite predictable incharacter. This type of construction is particularly suitable for porouswalls to be used as filters. v

It is sometimes desirable that the porosity of the resulting structurevary from one side to the other. This may be accomplished by alteringthe cross sectional configuration of the Wire as succeeding laminationsare wound on the mandrel. This may be done as the wire is wound bygradually or intermittently changing the adjustment of the flatteningrollers through which the wire passes. rlhus, the first laminations ofthe wire may be fiat as indicated by 7 in FIGURE 10 and succeedinglayers rendered less flat until the final layer may be square or roundas indicated by 8 in FIGURE 10.

It is, of course, not necessary in order to have relatively straightpore spaces that these pore spaces be normal to the surfaces of thefinished wall. By proper control of the rate of the feed the succeedinglaminations of wire may be displaced slightly so that the resulting porespace is essentially tangential as indicated by 9 in FlG- URES 11 and12. In this regard, it will be observed that in forming a cylinder theaxes of the pore spaces may be tangential as well as provided with aslope directed toward one end or the other of the cylinder.

As indicated hereinbefore, the mandrel on which the porous wall isformed may take the shape of the finished object providing such objectis a figure of revolution or an approximation thereof such as an ellipseor polygon.

Due to the initial bond between the wires it is possible to cut a porouswall cylinder or cone after sintering so as to form a flat sheet. Suchsheet may then be reformed as desired. It is not necessary, however, tocut the cylinder or other generated shape. Instead such member may bepressed into the desired final asymmetrical shape.

The metal comprising the wire is determined by the use for which theporous' wall is intended. Thus, steel, copper, titanium, molybdenumwires, as well as alloys, to mention only a few, may be employed to meetspecific conditions. Still further as shown in FIGURES 15 and 16, thewire may be plated or otherwise coated as indicated by 10. In this case,the bonding temperature need only be high enough to fuse the coatingmaterial.

For example, a copper plated steel wire would thus be capable of beingbonded or fused at a lower temperature than unplated steel wire.

It, of course, follows that in winding the wire back and forth acrossthe mandrel, the pitch must change to Zero and reverse at each end withthe result that there is a build-up at the extremities of the ligure ofrevolution. If the resulting decrease in porosity in these end regionsis objectionable in the final production, these portions may be trimmedoff after sintering. However, in many cases, these ends will be weldedor otherwise secured to other members, and the lack of porosity andcorrespondingly increased density and strength is advantageous.

It is necessary, of course, that the ends of the porous wall figurewound on the mandrel remain in place and not shift axially. If thewinding pitch angle is less than the frictional contact between the wireand the mandrel is sufficient to prevent axial shifting of the wire.

However, in winding the wire at pitch angles greater than 20 it isnecessary to pass the wire as a chord across the end of the mandrel. Thelocation of the chord path is such that the angle formed by thesubtended arc has a value equal to twice the pitch angle. By so windingthe ends of the gure forming the porous wall, the pitch angle may be 60or, if desired, even more. A 45 pitch angle will give the resultingporous wall figure equal strength in a longitudinal and acircumferential direction. If greater than 45 pitch angle is used, thestrength in a longitudinal direction is made greater than in acircumferential direction. Thus the strength characteristics of theporous wall may be adjusted to meet particular needs'.

While it is not necessary to wind over the end of the mandrel or over ashoulder thereon if the pitch angle is less than 20, it has been foundthat the uniformity is improved if this is done, particularly in thewinding of large diameter figures.

Having thus described certain embodiments and applications of myinvention, I do not desire to be limited thereto, but intend to claimall novelty inherent in the appended claims.

Iclaim:

1. A porous wall structure having opposed faces and comprising: amultiplicity of similar pairs of laminations of parallel spacedflattened wires, each lamination being the thickness of a single wireand wires of adjacent laminations being in angular relation andextending across' the common sides of a plurality of wires of adjacentlaminations, the flat surfaces of adjacent laminations being bondedtogether at said cross points, the spaces between wires in each of theinternal laminations forming channels parallel to but between theopposed faces of said wall structure and the intersections of saidchannels forming ports extending between and through the said faces ofsaid wall structure.

2. A porous wall structure as set forth in claim 1, wherein: said portsare relatively straight and continuous from face-to-face of said Wallstructure.

3. A porous wall structure as set forth in claim 1, wherein: the wiresof successive laminations are staggered and said ports form with saidchannels labyrinth passages through said wall structure.

4. A porous wall structure as set forth in claim 1, wherein: the ratioof wire width and channel width in said successive laminations variesfrom one side of said wall structure to the other.

5. A porous wall structure having opposed faces, cornprising: amultiplicity of pairs of laminations, each lamination being formed 0f aseries of flattened wires disposed in predetermined spaced, parallelrelation to each other and to the faces of the wall structure, saidwires defining therebetween a multiplicity of passages which are alsoparallel to each other and to the faces of said wall structure; theWires and passages of each lamination being disposed transversely to thewires and channels of adjacent laminations, with the flat sides ofsuperimposed crossing wires in abutment, said wires being bondedtogether at said abutting flat sides; those areas wherein the passagesof one lamination crossl the passages of an adjacent lamination deningports communicating with said passages and extending between andintersecting the opposed faces of said wall structure.

6. A porous wall structure, comprising: a multiplicity of pairsl oflaminations, each lamination including a multiplicity of parallel wiresforming continuous channels therebetween and defining a common surface,the wires of one lamination of each pair disposed in a transverserelation with those of the other lamination, each pair of laminationsdefining at the cross portions of said channels a multiplicity ofuniformly spaced ports extending through said pair of laminations; saidpairs of laminations being stacked to form compositely a wall structurewherein said channels are parallel to and within the external surfacesof said wall structure and said ports extend between the externalsurfaces thereof.

7. A porous wall structure as set forth in claim 6, wherein: said portsof adjacent pairs of laminations are in alignment thereby to formcontinuous ports extending between the external surfaces of said wall.

8. A porous wall structure as set forth in claim 6, wherein: said portsof adjacent pairs of laminations are in staggered relation thereby toform labyrinth passages incorporating both said ports and channels andcommunieating between the external surfaces of said wall.

9. A porous wall structure as set forth in claim 6 wherein: the ratio ofchannel width to wire width of said pairs of laminations varies betweenthe external surfaces of Said wall.

References Cited in the file of this patent UNITED STATES PATENTS1,071,822 Storey Sept. 2, 1913 2,082,513 Roberts June l, 1932 2,271,662Rubissow Feb. 3, 1942 2,327,184 Goodloe Aug. 17, 1943 2,485,827 HartzellOct. 25, 1949 2,711,828 Webb June 28, 1955 2,925,650 Pall Feb. 23, 1960FOREIGN PATENTS 324,924 Great Britain Feb. 3, 1930 OTHER REFERENCES NACAResearch Memorandum, Wire Cloth as Porous Material for TransportationCooled Wall, Nov. 13, 1951, by E. G. Eckert, National Advisory Committeefor Aeronautics, NACA RM E 51H23, pages y1l-22.

1. A POROUS WALL STRUCTURE HAVING OPPOSED FACES AND COMPRISING: AMULTIPLICITY OF SIMILAR PAIRS OF LAMINATIONS OF PARALLEL SPACEDFLATTENED WIRES, EACH LAMINATION BEING THE THICKNESS OF A SINGLE WIREAND WIRES OF ADJACENT LAMINATIONS BEING IN ANGULAR RELATION ANDEXTENDING ACROSS THE COMMON SIDES OF A PLURALITY OF WIRES OF ADJACENTLAMINATIONS, THE FLAT SURFACES OF ADJACENT LAMINATIONS BEING BONDEDTOGETHER AT SAID CROSS POINTS, THE SPACES BETWEEN WIRES IN EACH OF THEINTERNAL LAMINATIONS FORMING CHANNELS PARALLEL TO BUT BETWEEN THEOPPOSED FACES OF